Define the premise of your book in 25 words or less. This is the mission statement of your project and should be posted where you can easily see it while you are writing.
In a nonfiction book, the mission statement explains what the readers will learn --- for example: "This book teaches self-defense skills to people who are in wheelchairs."
In a work of fiction, the sentence is a summary of the core conflict. Example: A successful woman in her 50's learns that she was switched at birth by someone who is now stalking her.
Identify the best genre and setting to deliver your story. Using the example of the woman and the stalker, the tone, pacing, language and structure of the book are contingent on whether your novel is a thriller, comedy, romance, SciFi, mystery or historical epic. Each genre has its own set of protocols in order to be acceptable to publishers and it is critical to study what these rules are.
Make a list of all the characters in your book. Include physical descriptions, ages, occupations, relationships to other characters and what their stakes are in the outcome of the core conflict. Determine which character has the strongest presence to offer the primary point of view in action or narration. For a nonfiction book, make a list of chapter topics and determine the most logical order in which to present them.
Generate a list of things you already know about the subject matter of your book. Make a second list of what information you need to research in order to supplement your existing knowledge. If practical, try to assemble all of your research materials and notes prior to writing so as not to break your concentration or momentum.
Draft a three-act outline of your book that follows traditional structure of a beginning, middle and ending. Briefly list characters, key scenes and turning points that appear within each act. Equitably parse out the action to avoid a meandering opening, a sagging middle or a slap-dash resolution. Determine how to escalate the challenges for your characters and compel them to take greater risks.
Set a reasonable work schedule and projected timetable for writing and completing your book. This can either be a daily page quota or a monthly benchmark such as "four chapters by the 15th of each month."
Start writing.